AD7982 differential conversion single-ended signal circuit design

Many applications require high-resolution, differential input ADCs to convert single-ended analog signals, whether bipolar or unipolar. This DC coupling circuit can convert a single-ended input signal into a differential signal and is suitable for driving the 18-bit, 1 MSPS device AD7982 in the PulSAR series ADC. Using the ADA4941-1 single-ended to differential driver and the ADR435 ultralow-noise 5.0 V reference, this circuit can accept many types of single-ended input signals, including bipolar or unipolar signals in the high-voltage to low-voltage range.

The differential input voltage range of the AD7982 is set by the voltage on the REF pin. When VREF = 5 V, the differential input voltage range is ±VREF = ±5 V. The voltage gain (or attenuation) from the single-ended source VIN to OUTP of the ADA4941-1 is set by the ratio of R2 to R1. The ratio of R2 to R1 should be equal to the ratio of VREF to the peak-to-peak input voltage VIN. When the single-ended input voltage is 10 V peak-to-peak and VREF = 5 V, the ratio of R2 to R1 should be 0.5. The signal on OUTN is the inversion of the OUTP signal. The absolute value of R1 determines the input impedance of the circuit. The feedback capacitor CF is selected based on the required signal bandwidth, which is approximately 1/(2πR2CF). The 20 Ω resistor and 2.7 nF capacitor form a 3 MHz single-pole low-pass noise filter. Resistors R3 and R4 set the common-mode voltage at the IN input of the AD7982. The figure below shows the single-ended to differential DC coupled driver circuit diagram.